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Tuesday, July 3, 2012

The hummingbird that wasn't

Our daughter Ellen and her husband were out walking on Saturday near a local swimming hole and they spotted two gorgeous moths feeding at milkweed, though at first glance they didn't look like moths at all, but hummingbirds. It turns out they were Hummingbird Clearwing Moths, Hemaris thysbe, and as you can see in Ellen's stunning photos, they do a pretty good hummingbird mimic indeed.

This moth, a member of the sphinx moth family, ranges from as far north as the Yukon to at least as far south as the Amazon. It's often mistaken for a hummingbird because it's wings beat so fast, and, like a hummingbird, it flits from flower to flower, hovering rather than resting on the flower while sipping nectar. Click on the 3rd photo and you'll see the curled tongue that the moth dips deep into the flower, as it hovers in the air taking aim.

The usual explanation for one species so closely resembling another -- and there are many many examples -- is that there's an adaptive advantage for the mimic in looking like the species it's mimicking, the model. Usually it's that the model species is poisonous, or tastes bad or has spines or something that makes a predator think twice about eating it, and thus by association about eating the mimic This is called "Batesian mimicry," after English naturalist Henry Walter Bates who explored the Amazon in the mid-1800's and was fascinated by all the mimics he saw, particularly among butterflies. Bates read his paper proposing his antipredator theory of mimicry to the Linnean Society in 1861 (the same society before which Darwin and Wallace's theories about evolution were first announced to the world in 1858). Cases where such suggested selective pairings have been identified have reinforced the generic explanation.

And indeed the hummingbird moth is often mistaken for a hummingbird -- Bates himself wrote in his book The Naturalist on the River Amazons in which he describes his decade of explorations along the Amazon, that he often had trouble distinguishing between a hummingbird moth and a hummingbird, often shooting the former when he thought he was shooting the latter. One can imagine scenarios whereby resembling a hummingbird is advantageous because hummingbirds have relatively few predators and so looking like one would perhaps be safer than looking like a tasty moth. However, hummingbirds do have predators, including other birds, snakes, even frogs and insects -- e.g., see this rather astonishing story of hummingbird as prey. Bates didn't think the moth's resemblance to the hummingbird was adaptive. He attributed it instead to the fact that they occupied the same niche, hovering before trumpet-like flowers to drink their nectar.

Maybe so, but then why don't bees or butterflies who sip at the same flowers resemble hummingbirds? And the moth survives in such a large geographic range that it clearly manages just fine in many different niches. It feeds on many flowers where hovering would not be necessary, and at least in those places there wouldn't necessarily be selective pressure either for hovering or mimicry. So, it's not clear that Bates was right that its ecological niche gave the moth its hummingbirdness.

The novelist Vladamir Nabokov was a very learned lepidopterist in his day and was himself intrigued by mimicry. He organized the butterfly collection at the Museum of Comparative Zoology at Harvard -- recently his theory that the Polyommatus blue butterflies came to the New World across the Bering Straits in five waves was confirmed genetically.

He, too, was interested in the similarity between hummingbird moths and hummingbirds, although like Bates he didn't believe there was a functional explanation. Nor did he believe the similarity was due to natural selection at all, but instead that it was incidental. Even accidental. And it's hard to argue with that.

Just because an adaptive scenario is plausible doesn't make it true. That is, just because an organism does something, and hence is adapted to the environment where it does it, is no proof that the behavior was the result of natural selection for that behavior -- nor, of course, is it an argument against such an hypothesis.

One might not think that the hovering these moths do was likely to arise by chance, but there are thousands of lepitdopteran species, and a lot of traits can arise even if rarely, by chance. Since the moths don't particularly look in terms of coloration like humingbirds, and in fact are variable, since all sorts of other color adaptations have been suggested for moths and butterflies, and since mimicry may be coincidental if hovering were the adaptive cause, we lack a sound explanation of their appearance.

At the same time, such doubts don't detract from the interest, much less the beauty, of this product of evolution, however it came about.

I would say no, but I don't really know enough to be specific. What does it take for real imitation to be favored by selection?

Generally, it relates to predators and prey. It could also apply to mate choice--individuals looking like some other individual in order to get a mate.

Since there may be many reasons that physical appearance (including sound, smell, visual, etc.) could deceive a predator to avoid you 1. because you taste good but something you imitate tastes bad, 2. or for a predator to look like something harmless so the prey wouldn't get away, 3. or for a pattern to make an individual's parts or orientation look different from what it was (e.g., eye spots that make a prey species look as if its head--and its direction of escape motion--is at one end when it's really at the other, 4. Or to look fiercer or bigger or smaller or slower or faster than you really were,5. Or who knows what else?

....then I'd say there would be no area in which mimicry of various forms _might_ not be possible.

Plants don't prey on one another the way animals do on each other or on plants, but their appearance could at least confuse predatory animals.

Even bacteria and probably viruses can mimic the host's cells so the host's immune system doesn't recognize them.

At the same time, the world is filled (mainly!) with species that don't seem to be using mimicry, even if it's possible in principle doesn't mean it always occurs.

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